Scientists hail magnetic clue to planets' past

A belch of hot rock on the lunar surface 4bn years ago might throw new light on the making of planets and explain the so-called "man in the moon". The dark shapes of the moon's "seas" have tantalised observers for 10,000 years.

The shapes have been interpreted by various cultures as a woman weaving, stands of laurel trees, an elephant jumping off a cliff, a girl with a basket on her back, a rabbit, the lunar intestines spilled out on its surface after evisceration by a flightless bird, a woman pounding cloth, and even a four-eyed jaguar.

But geophysicists at the University of California, Berkeley, believe that some slow magma movement early in the moon's history could explain why Apollo astronauts found magnetised rocks on its surface 30 years ago.

All rocky planets must once have been hot. Mars and the moon are too small to maintain a hot interior.

But, using a sophisticated computer model of the moon's heart, Dave Stegman and colleagues modelled a blob of dark basalts rich in thorium early in lunar history, moving from the centre of the moon to the side now facing Earth.

This would have oozed into the blanket of insulation at the moon's crust, allowed a period of rapid heat flow and convection, and left behind a faint magnetic field and the magnetised rocks.

"If this model is correct, this would be the first full understanding of the thermal history of any planet, including the Earth, and would be a cornerstone for understanding the histories of all other planets," Mr Stegman said.